«Prepared for: Massachusetts Office of Coastal Zone Management 251 Causeway Street, Suite 800 Boston, MA 02114-2136 Prepared by: Applied Coastal ...»
SOUTH SHORE COASTAL HAZARDS CHARACTERIZATION ATLAS
DESCRIPTION OF VARIABLES
Massachusetts Office of Coastal Zone Management
251 Causeway Street, Suite 800
Boston, MA 02114-2136
Applied Coastal Research and Engineering, Inc.
766 Falmouth Road, Suite A-1
Mashpee, MA 02649
John S. Ramsey Sarah F. Griffee Mark S. Osler Mark R. Byrnes December 30, 2005 This publication was produced for the Massachusetts Office of Coastal Zone Management pursuant to the National Oceanic and Atmospheric Administration Award No. NA04NOS4190043.
The views expressed herein are those of the authors and do not necessarily reflect the views of NOAA or any of its sub-agencies.
TABLE OF CONTENTSA. INTRODUCTION
B. TIDE RANGE
C. WAVE CLIMATE
D. STORM SUSCEPTIBILITY
E. PROPERTIES WITH MULTIPLE FEDERAL FLOOD INSURANCE CLAIMS..............12 F. HISTORICAL SHORELINE CHANGE RATE
G. LITTORAL CELLS
H. SHORELINE TYPE
I. DOMINANT COASTAL PROCESSES
J. COASTAL ENGINEERING STRUCTURES
K. BEACH WIDTH FRONTING COASTAL BANKS
L. RELATIVE SEA-LEVEL RISE
LIST OF FIGURESFigure 1. Atlantic coast characteristics (from the U.S. Army Corps of Engineers, 2002).
Figure 2. Tide Range - Mean tide ranges along the Massachusetts coast
Figure 3. Wave Climate - Wave roses for two offshore wave hindcast stations showing the east-dominated wave conditions for the open ocean (Station
94) and more northeast dominated wave conditions for shorelines sheltered by Lower Cape Cod (Station 93). The wave roses divide wave data into direction bands and color code by wave height. The data is plotted radially by percent occurrence, which is labeled in the left portion of the rose. The wave data is from 1976-1995.
Figure 4. Historical hurricane tracks impacting Massachusetts from 1858 to 2000.
.............9 Figure 5. 100-year coastal storm surge elevations along the Massachusetts shoreline (derived from Tidal Flood Profiles, New England Coastline. U.S.
Army Corps of Engineers, New England Division, September, 1988).................10 Figure 6. Difference between 1-year and 100-year coastal storm surge elevations along the Massachusetts shoreline (derived from Tidal Flood Profiles, New England Coastline. U.S. Army Corps of Engineers, New England Division, September, 1988).
Figure 7. Plots of mean annual relative land levels.
Beneath the station names are the mean annual change of relative land level in mm/yr, and the confidence interval of the regression line, which has been drawn through the data points. From Emery and Aubrey (1991).
Figure 8. Beach profile plotted from LIDAR data in Plymouth showing a beach slope of approximately 1/10.
The Mean Seal-level (MSL) for 2005 and 2105 (predicted) are shown for reference.
A. INTRODUCTIONThe primary goal of the South Shore Coastal Hazards Characterization Atlas is to present information that can aid project review in areas where these projects may be vulnerable to coastal hazards. It is anticipated that the Atlas will assist local reviewers with the identification of technical information necessary to evaluate individual projects and implement sound coastal hazard mitigation strategies. Three major tasks were undertaken in the creation of the atlas: (1) compilation and review of existing data, (2) creation of a classification system for a range of coastal variables applicable to the entire coast of Massachusetts, and (3) preparation of GIS data layers and maps to present the information in a usable format.
Characterization of coastal hazards is not a new concept. Recently, the U.S. Geological Survey (USGS) developed relative coastal vulnerability assessments (e.g., Hammar-Klose et al., 2003 and Thieler and Hammar-Klose, 1999). Although these large-scale assessments are useful, the unique geological and geographic setting of the Massachusetts coast requires a region-specific analysis. In addition, contemporary analysis techniques (e.g., GIS, rectified aerial photography, and digital topographic information) allow for more accurate delineation of coastal features, as well as other variables that help characterize local coastal hazards. It is noted that this analysis was performed for open coasts facing Cape Cod or Massachusetts Bays. In addition, the protected shoreline along Kingston Bay, Duxbury Bay, and Plymouth Harbor certain relevant variables were mapped (Shoreline Type, Shoreline Change Rate, and Repetitive Loss).
The unique geologic and geographic setting of the Massachusetts shoreline consists of a variety of shoreline types, ranging from bedrock outcrops to barrier beach systems. In addition, the geographic location of Massachusetts and the variable orientation of its shoreline make different portions of the coast susceptible to major damage from both tropical storms (hurricanes) and extra-tropical storms (northeasters). As indicated in Figure 1, the Massachusetts shoreline is characterized by glacial deposits, many forming high banks punctuated by areas of lower-lying beach. Massachusetts represents one of the few shorelines along the U.S. East Coast where glacial moraine or drumlin deposits dominate the regional sediment supply to beaches. Since the geologic and geographic setting of the Massachusetts coast governs how the shoreline changes over time, these characteristics need careful consideration in the assessment of coastal hazards.
Based on the unique characteristics of Massachusetts, and specifically the South Shore, the following is a list of variables used to characterize coastal hazards. The purpose for selecting these variables and the general method for evaluating each variable is described in the following sections of this guide.
• Tide Range
• Wave Climate
• Storm Susceptibility
• Properties with Multiple Federal Flood Insurance Claims
• Historical Shoreline Change Rate
• Littoral Cells
• Shoreline Type
• Dominant Coastal Processes
• Coastal Engineering Structures
• Beach Width Fronting Coastal Banks
• Relative Seal-level Rise Figure 1. Atlantic coast characteristics (from the U.S. Army Corps of Engineers, 2002).
B. TIDE RANGE Purpose: To illustrate the variations in tide range along the Massachusetts coastline.
Methodology: The tide ranges along the coast of Massachusetts were obtained from NOAA’s Center for Operational Oceanographic Products and Services (CO-OPS). Tide range is defined as the difference between Mean Low Water (MLW) and Mean High Water (MHW).
The tides in Massachusetts are semidiurnal, meaning that in each period of 24 hours and 50 minutes (just over a day), there are two high and low tides. The tide range around the Massachusetts coast varies greatly, from less than 2 feet (0.6 m) near Woods Hole and Falmouth to greater than 9 feet (2.7 m) in Cape Cod and Massachusetts Bays (Figure 2). It is important to keep in mind that these ranges are for the astronomical tides only. Any contribution to local water level change due to storm or wave effects is not included. Furthermore, these tide ranges are averages, and the actual tide range for a given location changes gradually from day to day. During the new and full moons, the tide range will be the greatest, the so called “spring” tides. During a spring tide, a location with a 9 foot (2.7 m) average tide range can see a tide range in excess of 12 feet (3.7 m). Between the full and new moons, when the moon is half full, the tide range is the smallest and this is referred to as the neap tide. So, throughout the month, the tide at a given location is in constant flux, either increasing towards the spring tide range or decreasing towards the neap tide.
It should be noted that around the Chatham area there is a nodal point in the tides, where the average range changes significantly over a relatively small distance. Along the southern coast of Cape Cod, the average tide range increases gradually from less than 2 feet (0.6 m) at Falmouth, to about 3 feet (0.9 m) south of Cotuit to almost 4 feet (1.2 m) just west of Monomoy Island. A tide range of about 4 feet (1.2 m) is also seen on the ocean side of Monomoy Island but then jumps sharply to 6 feet (1.8 m) or more offshore of Chatham Harbor, which is only 10 miles (16 kilometers) or so north of Monomoy Point. From offshore of Chatham the tide range increases gradually from 6 feet (1.8 m) to more than 8 feet (2.4 m) northeast of Provincetown.
For the South Shore, typical northeast storms impacting the coastline have a duration longer than a tide cycle; therefore, tide range is not directly related to susceptibility to storm damage. Damage typically occurs during conditions of elevated water levels beyond the normal high tide levels (e.g. during periods of storm surge). However, for relatively short-duration tropical storms (e.g., hurricanes), a large tide range can decrease the storm impacts. Since hurricanes typically pass through Massachusetts in a matter of a few hours, a large tide range can prevent storm surge from causing flooding problems, if the storm passage is between low and mid-tide levels.
Figure 2. Tide Range - Mean tide ranges along the Massachusetts coast.
C. WAVE CLIMATEPurpose: To illustrate the direction and amount of wave energy that impacts the South Shore coast. This information can be used as a first step in determining the dominant driving force for moving sediment along the beaches of the South Shore.
Methodology: The U.S. Army Corps of Engineers Wave Information Study (WIS) provided the information for this variable. Wave height and direction information were summarized in a “wave rose” that indicates the percent occurrence and height of waves influencing the coast.
The wave climate along the South Shore is dominated by waves from the east and northeast. The wave roses shown in Figure 3 are compiled from data taken from the U.S. Army Corps of Engineers Wave Information Study (WIS). This study uses historical meteorological data to calculate hourly wave conditions, which are then verified against measurements from wave buoys. The resultant data set is comprised of 20 years (1976-1995) of hindcast wave information, including significant wave height, peak period, and direction once each hour.
The wave roses in Figure 3 show the percentage of waves that arrive from a given directional band and the distribution of wave height within that direction band. The median direction of wave incidence for Station 94 is 79 degrees (east-northeast) and the average wave height of the highest 5% of waves is 12.4 feet (3.8 m). For Station 93 to the south, the median incident direction is 29 degrees (north-northeast), with the highest 5% of waves averaging 9.9 feet (3.0 m). These wave roses show that the coastline in the northern part of the study area is subject to more severe wave attack when compared with the southern portions of the study area. In addition, the northern section of the study area is impacted by waves primarily from the east and east-northeast directions, while the southern section is dominated by waves from the northeast. The difference in incident direction is an important factor to understand, as Cape Cod provides significant sheltering to the southern reaches of the study area. Cape Cod serves to block open ocean waves approaching much of the South Shore from the east, southeast, and south. The frequent occurrence of waves from the east and east-northeast at Station 94 is best explained by the lack of sheltering from Cape Cod. At this site, the dominant easterly waves from the North Atlantic Ocean are allowed to propagate unimpeded toward the shoreline.